How do you remove the heater core on a 99 Expedition? has a very helpful tutorial here:

Search google and you will find a few very good videos on how to do this.

I thought my core was going bad - got the stinky smell from my AC vents - did some looking here on the web and found a tip to turn on the heat - that confirmed it for me as the huge billowing mist or smoke came out - called pep boys just now - they want 1000 bucks to fix - 721 for labor - 150 for AC dump and recharge (not sure this is needed) and then 72 bucks for the part. Good luck.